Cytochrome P450 (P450) enzymes play crucial roles in the clearance of drugs from the circulation, and therefore changes in their activities can significantly influence the therapeutic and adverse effects of a drug on an individual. In addition some P450 enzymes have crucial roles in metabolism of physiological substrates e.g. CYP2C enzymes in the metabolism of arachidonic acid to bioactive epoxyeicosatrienoic acids. Infectious and inflammatory diseases cause the down-regulation of many P450s and other drug metabolizing enzymes in the liver, causing impairment of drug clearances. Our laboratory has shown that nitric oxide (NO) formed in hepatocyte during an inflammatory response selectively directs the ubiquitin-dependent proteasomal degradation of rat CYP2B1 and CYP3A2, but not CYP2C11. Our preliminary data demonstrate that not only are human CYP2B6 and CYP2C enzymes down-regulated by NO, but they are more sensitive than the rat enzymes to degradation caused by NO-releasing chemicals. Here, we propose to use proteomic methods to define the scope of NO-mediated degradation in human hepatocytes. We will characterize the proteolytic enzymes involved in the NO-mediated degradation of CYP2B6 and 2C9, and elucidate the mechanism whereby NO regulates these processes in an enzyme-specific manner